Seasonal Climate Outlook Rainfall Archive

Potential spring relief for southwest W.A.

Rainfall odds for spring 2001 raise the prospect of good
rains in southwest and southern W.A. These areas have suffered below
average falls in both 2000 and 2001, but an abnormally warm Indian
Ocean gives the region a 60 to 70% chance of exceeding the median
spring rainfall (see map below), according
to the National Climate Centre.

So under this climate pattern, about 6 or 7 spring seasons out of
10 are expected to be wetter than usual here, whilst about 3 or 4 out
of 10 are drier. In addition, the outlook scheme has moderate to high
levels of skill in southwest W.A. for spring.

In most other parts of the country, the current ocean patterns do
not strongly favour either above or below average rainfall according
to the statistics used by the National Climate Centre. In other words,
chances of rainfall exceeding the median are fairly close to the
baseline level of 50%.

Background Information:

The Outlook probabilities are based on recent
Indian and Pacific Ocean temperatures. There was little change
in the eastern tropical Pacific in July, but an abnormally warm
Indian Ocean became even warmer, and this dominates the rainfall
outlook.

Continued neutral conditions is the most likely outcome for the
tropical Pacific during the remainder of the year. Only four of twelve
international computer
models predict an El Niņo by late this year or early 2002.

The July Southern Oscillation Index
(SOI) value was -3, a marginal drop from June's +2 but a persistence of neutral
values nevertheless. The approximate SOI for the 30 days ending 12th August
was -3.

This outlook represents a summary, more detail is available from the
contact people or from
SILO.

Important: Probability outlooks should not be used as if they were
categorical forecasts. More on probabilities is contained in the booklet
"The Seasonal Climate Outlook - What it is and how to use it",
available from the National Climate Centre.

More information on this outlook is available during normal office hours
from 9:00am to 5:30pm (EST) Monday to Friday by contacting the following climate
meteorologists in the National Climate Centre:

A:General statements about the probability or risk of wetter or drier than
average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken
from rainfall and sea surface temperature records. They are not, however, categorical predictions about
future rainfall, and they are not about rainfall within individual months of the three-month outlook period.
Q: WHAT DO WE MEAN BY "WETTER OR DRIER THAN AVERAGE, OR "WARMER OR COOLER THAN AVERAGE""?

A:Being above or below the median rainfall, average
maximum temperature, or average minimum temperature for the three-month period.
The median is a useful measure of "normal" rainfall. In the long term,
rainfall is above median in one half of years, and below median in the other half.
For example, from July to September at Mackay in Queensland, one-half of 3-month rainfall totals have been
below 80mm, and one-half have been above. If rainfall was above 80mm in that period it would be "wetter than
average" or above median. Over the long haul there is a 50% chance of this occurring.
In terms of odds this is even money.
Note that the average maximum temperature is the average of all the daily highest temperatures for the period.
Similarly, the average minimum temperature is the average of all the daily lowest temperatures for the period
Q: HOW ACCURATE ARE THE OUTLOOKS?

A: In the places and seasons where the outlooks are most skilful,
the eventual outcome (above or below median) is correctly given the higher chance about 70 to 80% of the
time. In the least skilful areas, the outlooks perform no better than random chance or guessing. The
rainfall outlooks perform best in eastern and northern Australia between July and January, but are less
useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature
peaks in early winter and drops off marginally during the second half of the year. The lowest point in
skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn
and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer
and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk.
For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the
original assessment of a 50:50 chance could still have been correct.
Q: WILL CATEGORICAL OUTLOOKS EVER BE ISSUED? (Eg. It WILL be drier than average.)

A: Very unlikely. There is a certain level of natural variability in the climate
which is chaotic and unpredictable. This is particularly the case with rainfall. For example, rainfall in a season can
be significantly above average in one region, and significantly below average less than 50km away.
Q: HOW SHOULD THE OUTLOOKS BE USED?

A: As another tool in risk management and decision making. The benefits accrue
from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several
years, the advantages of taking account of the risks will outweigh the disadvantages. For more information on the use
of probabilities, farmers could contact their local departments of agriculture or primary industry.

The SOI is one indicator of the stage of El Niņo or La Niņa events
in the tropical Pacific Ocean. It is best considered in conjunction with
sea-surface temperatures, which form the basis of the outlooks.

A strongly negative SOI (below -10) is characteristic of El Niņo, which is often associated
with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north.

A strongly positive SOI (above +10) is characteristic of La Niņa, which is often associated with
above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start
to the northern monsoon season.
El Niño &
La Niña

El Niņo translates from Spanish as "the boy-child", and refers to the extensive warming
of the central and eastern Pacific Ocean.

La Niņa translates from Spanish as "the girl-child",
and refers to the extensive cooling of the central and eastern Pacific Ocean. The term has
recently become the conventional label for the opposite of El Niņo.